Science —

Increased deoxygenation of the oceans

New data collected from floating sensors reveals that large regions of the …

70 percent of the planet's surface is covered in water, and the oceans contain the bulk of our planet's biomass and biodiversity. It is true to say that even now, in 2008, we have a limited understanding of just exactly what is out there in our seas and oceans; even sailing around on a yacht running sea water through a sequencer results in new discoveries.

Sadly, as we've covered before, the oceans are not currently a happy place. Environmental contamination, be it heavy metals such as mercury, floating plastic detritus covering hundreds and thousands of square miles, or the growing acidification of the oceans, is getting worse, with knock-on effects for marine life and the humans whose livelihoods depend on the seas.

One frequent concern is the state of oxygenation of the oceans. Although mammals aren't capable of gas exchange in water, pretty much everything else in the sea is, but only when the amount of oxygen dissolved in that water is sufficiently concentrated. Unfortunately, a number of factors can affect this, from algal blooms that use up the available oxygen, to warming oceans and altered currents, the result can be hypoxia (low oxygen) or even anoxia (no oxygen) in some regions, where the amount of dissolved O2 cannot support macroorganisms such as fish.

The current issue of Science features a paper on the changing O2 levels in the ocean, using data collected between 1960 and the present. The data were gathered from Argo floats that have been equipped with O2 sensors. The floats are part of a network of 3000 such devices that report, via satellite, on ocean conditions such as temperature and salinity over a vertical range of 2000 m.

The data returned from these floats paints a gloomy picture: over more than four decades, there are growing hypoxic zones in the tropical Atlantic and Pacific, especially at depths between 300- 700 m. In these areas, the O2 concentration is falling by 0.09 to 0.34 µmolkg-1year-1, and these figures are consistent with climate models that show warming and deoxygenating oceans.

Obviously the implications for marine life are not good; with fish stocks already threatened beyond the point of sustainability, regions with insufficient O2 to support life might as well be on the moon. There are also implications for plans to capture atmospheric CO2 via seeding the oceans with iron filings. Here, some scientists have postulated it might be possible to dump large amounts of iron into the open ocean to stimulate plankton growth that would take up CO2, but a probable consequence would be a further deoxygenation of the water.